Pulse-echo acoustic ranging systems, also known as time-of-flight ranging systems, are commonly used in level measurement applications. Pulse-echo acoustic ranging systems determine the distance to a reflector (i.e. reflective surface) by measuring how long after transmission of a burst of energy pulses the echoes or reflected pulses are received. Such systems typically use ultrasonic pulses, pulsed radar or microwave signals.
Pulse-echo acoustic ranging systems generally include a transducer and a signal processor. The transducer serves the dual role of transmitting the energy pulses and receiving the reflected energy pulses or echoes. An echo profile is generated from the received energy pulses. Echo pulses are identified in the echo profile by the signal processor, and the distance or range of the object is calculated based on the transmit times of the transmitted energy pulses and the receive echo pulses.
To provide accurate level measurements, the echo pulses must be precisely detected and processed. One approach involves transmitting multiple transmit pulses, i.e. “shots”, and averaging the echo positions associated with the multiple transmit pulses. For noisy environments, for example, a waste water tank, the number of shots is increased for the average in an effort to reduce the effects of noise. The aim of this approach is to dampen or smooth random deviations in the echo pulse position in an echo profile. However, the technique does not prevent noisy profiles from becoming part of the average. In a noisy environment, such as filling a waste water tank, the level measurements can be very imprecise because the average echo-position is based on a large number noisy echoes.
The commonly used technique for finding echoes in an echo profile involves generating a time varying threshold or TVT curve. The TVT curve provides a baseline or line on the echo profile which is above the noise level in the echo profile. Valid echoes appear above the TVT curve. Various algorithms and techniques are known in the art for determining the noise floor and generating the TVT curve.
Two solutions for improving level measurements for echo-based systems in the presence of noise comprise; (1) increasing the number of shots taken and averaging the results; and (2) measuring the noise floor and using the average noise level to generate a TVT curve to assist in the interpretation of the echo profile curve.
A typical echo profile indicated by reference 100 is shown in FIG. 1 together with a TVT curve indicated by reference 120. The first portion of the echo profile 100 comprises a half pulse 140 which corresponds to the ring-down in the transducer. The ring-down compris s the period or interval in which the transducer is still “ringing down” from the transmit pulses emitted and as such this interval is not considered for detecting reflected energy pulses. Following the ring-down 140, the echo profile 100 comprises a number of pulses 160, indicated individually as 160a, 160b, 160c, 160d and 160e, in FIG. 1. Using the TVT curve 120, the pulses 160a, 160b, 160c and 160d are identified as valid receive echo pulses. The portion of the echo profile 100 that falls below by the TVT curve 120 is considered to be noise and in FIG. 1 is indicated generally by reference 150. For instance, the last pulse 160e falls below the TVT curve 120 and is considered to comprise noise.
While the TVT technique has been used successfully in level measurement and time-of-flight ranging systems, there are shortcomings. First, generating the noise floor and the TVT curve can be a processor intensive process. Secondly, most TVT curves require manual adjustments to provide the best performance, and thirdly different TVT curves will work better in some situations rather than others.
Accordingly, there remains a need to provide a system and techniques which improve the processing of the reflected energy pulses or echoes.